1. Technical Field
The present disclosure relates to a nitride semiconductor device that is able to be applied to a power transistor used by a power source circuit of consumer equipment, such as a television.
2. Description of the Related Art
Nitride semiconductors represented by GaN are so-called wide band gap semiconductors, the band gaps of GaN and AlN are greater than 3.4 eV and 6.2 eV at room temperature, respectively. Nitride semiconductors have a large dielectric breakdown field and have a saturated drift velocity of electrons greater than compound semiconductors such as GaAs or Si semiconductors and the like. Therefore, experimental development of power transistors used in nitride semiconductors which are advantageous in increasing output and increasing breakdown voltage is currently being actively performed.
In an AlGaN/GaN hetero structure, a high concentration of two-dimensional electron gas (2DEG) occurs at the heterointerface due to spontaneous polarization and piezoelectric polarization on the C-plane ((0001) plane). A sheet carrier density of 1×1013 cm−2 or more is obtained in the two-dimensional electron gas even when undoped.
AlGaN indicates a three-dimensional mixed crystal AlxGa1-xN (x is a given value, wherein 0≤x≤1). Below, for the multidimensional mixed crystal, the arrangement of the respective constituent element symbols is abbreviated to, for example, AlInN, GaInN or the like. For example, the nitride semiconductors AlxGa1-x-yInyN (x and y are given values, wherein 0≤x≤1, 0≤y≤1) is abbreviated to AlGaInN.
FIG. 19 is a cross-sectional view of a field effect transistor in an example of the related art. In FIG. 19, AlGaN buffer layer 102 (layer thickness 0.5 μm), GaN drift layer 103 (layer thickness 4 μm), p-GaN barrier layer 104 (layer thickness 0.5 μm), and AlGaN cap layer 105 (layer thickness 0.3 μm) are formed in this order on conductive SiC substrate 101. An opening which reaches drift layer 103 is formed on cap layer 105, and GaN electron transit layer 106 (layer thickness 50 nm), AlN intermediate layer 107 (layer thickness 2 nm), and AlGaN electron supply layer 108 (layer thickness 30 nm) are formed in this order by regrowth. Ni gate electrode 109 is formed to cover the side surface of the opening, and Ti/Al source electrode 110 is formed on a flat section of AlGaN electron supply layer 108. Drain electrode 111 is formed on the rear surface of SiC substrate 101. A drain current flows from drain electrode 111 on the rear surface side of the substrate, through GaN drift layer 103, to source electrode 110 on the front surface side, and is a vertical transistor which opens and closes, through a gate voltage, the channel of the two-dimensional electron gas (2DEG) generated at the AlGaN/GaN heterointerface formed in the side wall of the opening.